Perovskite metal-oxides, such as BaxSr1-xTiO3 (hereinafter called “BST”), show exceptional material properties including (i) switchable permanent polarization (ferroelectric phase), (ii) piezoelectricity (ferroelectric phase), (iii) field-dependent (tunable) polarization (paraelectric phase), (iv) low losses at high frequencies (paraelectric phase). Tunability and low losses, typically obtained in the paraelectric phase, make these materials of special interest for high-frequency applications. An example of a basic device realizable using these materials is a tunable capacitor (varactor).
For optimal tunability, parallel-plate device geometry is highly desirable: here the perovskite metal-oxide is sandwiched between two (top and bottom) metallic electrodes. Using planar device geometries based on metal electrodes deposited only on top of the perovskite metal-oxide, it is difficult to obtain sufficient tunability.
In the prior art parallel-plate capacitor fabrication process, the layers are deposited successively on a substrate, ie. the metallic bottom electrode is first deposited on the substrate, then the BST layer is deposited on the metallic bottom electrode and finally the metallic top electrode is deposited on the BST.
High processing temperatures are required for proper crystallization of perovskite oxides (e.g. for BST, Tmin˜650° C.). The high temperature and the presence of oxygen make the parallel-plate device processing problematic. The metallic bottom electrode is heavily attacked during BST growth. Typical problems encountered in the bottom electrode are (i) electrode destruction via oxidation, (ii) grain growth resulting in poor BST quality. On the other hand, the electrode material should be well conducting to prevent the critical losses at high frequencies. Of typical metals, Cu, Ag, Al, and Au are well conducting but either get easily oxidized and/or are unstable substrates for BST growth due to their low melting point. A typical state-of-the-art choice for the bottom electrode is Pt, which, however, (i) is a significantly worse conductor than the metals above, (ii) will easily exhibit grain growth if thick layers are employed to reduce conductor losses. Furthermore, Pt is permeable with respect to oxygen, and reactive with respect to other metals—both effects complicating the fabrication of multilayered electrode structures. The refractory metals (Mo,W) are more stable against grain growth and they are intermediate conductors but get easily oxidized. Protective layers (e.g. diffusion barriers) can in principle be employed to protect the bottom electrode from oxidation, but it is difficult to obtain a well-protecting layer with minimal parasitic effects (e.g. non-tunable capacitance).